likely() and unlikely()
What are they ?
In Linux kernel code, one often find calls to likely() and unlikely(), in conditions, like :
bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx);
if (unlikely(!bvl)) {
mempool_free(bio, bio_pool);
bio = NULL;
goto out;
}In fact, these functions are hints for the compiler that allows it to correctly optimize the branch, by knowing which is the likeliest one. The definitions of these macros, found in include/linux/compiler.h are the following :
#define likely(x) __builtin_expect(!!(x), 1) #define unlikely(x) __builtin_expect(!!(x), 0)
The GCC documentation explains the role of __builtin_expect() :
-- Built-in Function: long __builtin_expect (long EXP, long C)
You may use `__builtin_expect' to provide the compiler with branch
prediction information. In general, you should prefer to use
actual profile feedback for this (`-fprofile-arcs'), as
programmers are notoriously bad at predicting how their programs
actually perform. However, there are applications in which this
data is hard to collect.
The return value is the value of EXP, which should be an integral
expression. The value of C must be a compile-time constant. The
semantics of the built-in are that it is expected that EXP == C.
For example:
if (__builtin_expect (x, 0))
foo ();
would indicate that we do not expect to call `foo', since we
expect `x' to be zero. Since you are limited to integral
expressions for EXP, you should use constructions such as
if (__builtin_expect (ptr != NULL, 1))
error ();
when testing pointer or floating-point values.
How does it optimize things ?
It optimizes things by ordering the generated assembly code correctly, to optimize the usage of the processor pipeline. To do so, they arrange the code so that the likeliest branch is executed without performing any jmp instruction (which has the bad effect of flushing the processor pipeline).
To see how it works, let's compile the following simple C user space program with gcc -O2 :
#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)
int main(char *argv[], int argc)
{
int a;
/* Get the value from somewhere GCC can't optimize */
a = atoi (argv[1]);
if (unlikely (a == 2))
a++;
else
a--;
printf ("%d\n", a);
return 0;
}Now, disassemble the resulting binary using objdump -S (comments added by me) :
080483b0 <main>: // Prologue 80483b0: 55 push %ebp 80483b1: 89 e5 mov %esp,%ebp 80483b3: 50 push %eax 80483b4: 50 push %eax 80483b5: 83 e4 f0 and $0xfffffff0,%esp // Call atoi() 80483b8: 8b 45 08 mov 0x8(%ebp),%eax 80483bb: 83 ec 1c sub $0x1c,%esp 80483be: 8b 48 04 mov 0x4(%eax),%ecx 80483c1: 51 push %ecx 80483c2: e8 1d ff ff ff call 80482e4 <atoi@plt> 80483c7: 83 c4 10 add $0x10,%esp // Test the value 80483ca: 83 f8 02 cmp $0x2,%eax // -------------------------------------------------------- // If 'a' equal to 2 (which is unlikely), then jump, // otherwise continue directly, without jump, so that it // doesn't flush the pipeline. // -------------------------------------------------------- 80483cd: 74 12 je 80483e1 <main+0x31> 80483cf: 48 dec %eax // Call printf 80483d0: 52 push %edx 80483d1: 52 push %edx 80483d2: 50 push %eax 80483d3: 68 c8 84 04 08 push $0x80484c8 80483d8: e8 f7 fe ff ff call 80482d4 <printf@plt> // Return 0 and go out. 80483dd: 31 c0 xor %eax,%eax 80483df: c9 leave 80483e0: c3 ret
Now, in the previous program, replace the unlikely() by a likely(), recompile it, and disassemble it again (again, comments added by me) :
080483b0 <main>: // Prologue 80483b0: 55 push %ebp 80483b1: 89 e5 mov %esp,%ebp 80483b3: 50 push %eax 80483b4: 50 push %eax 80483b5: 83 e4 f0 and $0xfffffff0,%esp // Call atoi() 80483b8: 8b 45 08 mov 0x8(%ebp),%eax 80483bb: 83 ec 1c sub $0x1c,%esp 80483be: 8b 48 04 mov 0x4(%eax),%ecx 80483c1: 51 push %ecx 80483c2: e8 1d ff ff ff call 80482e4 <atoi@plt> 80483c7: 83 c4 10 add $0x10,%esp // -------------------------------------------------- // If 'a' equal 2 (which is likely), we will continue // without branching, so without flusing the pipeline. The // jump only occurs when a != 2, which is unlikely. // --------------------------------------------------- 80483ca: 83 f8 02 cmp $0x2,%eax 80483cd: 75 13 jne 80483e2 <main+0x32> // Here the a++ incrementation has been optimized by gcc 80483cf: b0 03 mov $0x3,%al // Call printf() 80483d1: 52 push %edx 80483d2: 52 push %edx 80483d3: 50 push %eax 80483d4: 68 c8 84 04 08 push $0x80484c8 80483d9: e8 f6 fe ff ff call 80482d4 <printf@plt> // Return 0 and go out. 80483de: 31 c0 xor %eax,%eax 80483e0: c9 leave 80483e1: c3 ret
How should I use it ?
You should use it only in cases when the likeliest branch is very very very likely, or when the unlikeliest branch is very very very unlikely.